Injection molding machines are well known and commonly used to produce a wide variety of plastic articles. Generally, a material, such as a plastic resin in the form of pellets, is fed to the machine through a hopper, and thence to a plasticizer where it is melted. The molten resin then flows under pressure to a nozzle, is injected through a gate into a mold cavity, cooled to its "freezing" temperature, and ejected from the mold cavity to complete a single molding cycle.
One area in which improvements can be made in the injection molding field is reducing the cycle time, thereby increasing the number of articles that can be produced by a machine. The cycle time for an injection molding machine is determined by a number of interdependent factors, including the physical and chemical attributes of the resin, the size of the molded article, and the time the article cools in the mold before it is ejected.
As is apparent, reducing the time needed to cool the article in the mold will reduce the overall cycle time. However, undesirable physical defects often result from attempts to reduce the cooling time, particularly in articles, such as preforms, made from polyethylene teraphthalate (PET). The most common of these undesirable characteristics are gate defects that occur in that portion of the preform in the vicinity of the gate. These common defects include crystalline halos and plugs, gate voids, internal dimples, scarred gates, and sticky or stringy gates. Many variables affect the quality of the gate area in a finished preform. Processing parameters, such as mold gate timing, nozzle tip temperature, and the flow rate of cooling fluid can all be adjusted to improve preform quality. However, insufficiently rapid heat transfer at the gate area remains one of the most persistent difficulties to overcome, and a continuing obstacle to greatly improved cycle times.
In a typical hot runner injection molding system with valve gating, insufficient cooling in the gate area can be attributed to the several competing functions of the gate area, and the cyclic temperature swings to which it is subject. The gate is a passage, generally a tapered hole formed in gate insert that directs the flow of molten resin from the nozzle to the mold cavity. The mold gate insert acts as a locator for the nozzle tip on one side, and forms part of the mold cavity at its other side. Its nozzle side is subject to a constant high nozzle tip temperature that can be undesirably transferred through the insert to the mold cavity. Meanwhile, the mold cavity side of the gate insert must quickly cycle between a high temperature when the gate is open to a low temperature sufficient to freeze the resin when the mold has been filled and the gate closed.
Further, in valve gating, the opening and closing of the gate is achieved mechanically with a valve stem. This stem can be moved between an open position, permitting the flow of molten material through the gate, and a closed position where the valve stem seats in the gate thereby forming a seal and preventing molten material from passing through the gate. One disadvantage of valve gating is that the valve stem is in close proximity to the nozzle assembly. As a result, the valve stem tends to be very hot in comparison to the gate area of the mold, which can effect the quality of the sprue gate formed on a molded article. Undesirable properties such as crystalline sprues, and other defects, can result. Another disadvantage of valve gating is caused by the cyclic opening and closing of the gate which subjects the seating area, or gate land, to heavy wear by the valve stem.
To overcome some of the disadvantages of valve gating, in thermal gating the valve stem is eliminated. The gate is opened and closed by temperature cycling at the nozzle to freeze or heat the material in the gate area. However, the gate area is still subject to cyclic heating and cooling, and thermal isolation of the nozzle from the mold is a concern.
To provide acceptable thermal isolation of the mold cavity from the high nozzle tip temperatures, and wear resistance in the gate land, prior art gate inserts have generally had to compromise on the thermal conductivity, and hence the speed of heat transfer, of the gate insert material.
Several prior art references disclose thermal shielding at the nozzle tip to limit cooling of the hot runner nozzle tip in the vicinity of the mold gate area. For example, U.S. Pat. No. 3,741,704 to Beasley discloses a thermally insulating sleeve, made of a material such as asbestos, attached to the upper surface of the mold die. The sleeve prevents loss of heat from the hot nozzle tip to the cooler mold die, and is intended to prevent resin freeze up at the gate. Such an insulating sleeve can deteriorate due to wear caused by the valve stem and the passage of abrasive or corrosive molten materials. U.S. Pat. No. 4,268,240 to Rees et al. discloses a thermally insulating sheath formed by a plug of cold resin either before or during the molding process. Cleaning and maintenance, and changing resin properties are all adversely affected by such a means of thermal insulation of the nozzle tip.
U.S. Pat. No. 4,416,608 to Deardurff discloses several ways to thermally insulate a hot runner nozzle housing from a cooler mold. Deardurff discusses the formation of an air gap around the nozzle, providing an angle at the bottom portion of the nozzle housing to limit the contact between the nozzle and the mold, and providing a nozzle housing having at the tip a bottom portion having a rough surface in contact with the mold. Generally, Deardurff has limited application as it does not provide any sealing means at the gate area. Also, because of the direct contact between the nozzle tip and the mold gate area the gate cannot be efficiently thermally insulated, and crystallinity penetration at the sprue gate portion of PET preforms can occur during cooling of the cavity.
Reference is also made to U.S. Pat. No. 4,622,001 to Bright, which discloses a mold with water cooling channels at the mold gate. The channels are formed between a cap member attached between the nozzle tip and the mold cavity member. While the cooling taught in Bright can improve the cooling in the gate area of a molded article, it is apparent that it can cause undesirable cooling of the nozzle tip.
Some of the disadvantages of prior art nozzle tip insulation, as described above, includes the relative complexity of the various insulating sheaths, caps and sleeves. They are generally not resistant to either chemical or physical wear, and their replacement involves substantial disassembly and down time of the associated injection molding machinery. One solution has been to use mold gate inserts that can be more easily replaced when necessary. Mold gate inserts are well known in the art, as shown for example in U.S. Pat. No. 4,911,636 to Gellert and U.S. Pat. No. 5,652,003 to Gellert. The inserts form the upper portion of the mold cavity, and provide a seat for the nozzle tip. While providing some thermal shielding to the nozzle tip, these gate inserts are still subject to high nozzle tip temperatures and can still result in insufficiently rapid cooling of the mold gate area of a molded article, and result in defects as enumerated above.
It is therefore desirable to provide a novel gate insert that can be manufactured from materials with high thermal conductivity to permit more efficient heat transfer and reduced cooling time, without sacrificing wear resistance and thermal isolation from nozzle tip temperatures.